1. Field of the Invention
The present invention relates to an image forming apparatus, a density-shift correction method, and a computer program product.
2. Description of the Related Art
Higher speed operation is increasingly required of color copiers, and thus a mainstream of the color copiers is tandem-type color copiers in which electrostatic-latent-image forming units each including a photosensitive element are arranged in parallel to each other. The electrostatic-latent-image forming units are provided with four colors of cyan (C), magenta (M), yellow (Y), and black (K). An important task of the tandem type is a technology for registration between the colors in terms of its configuration.
Therefore, many of the tandem-type color copiers have a function of reducing color shift by forming a predetermined toner pattern with toner of each color on a transfer belt and detecting the toner pattern by using an optical sensor to calculate each amount of color shifts between relevant colors factor by factor and by performing feedback-correction on each color shift so as to eliminate each of the color shifts. The factor includes registration shifts, magnification shifts, and skew deviations in a main-scanning direction and a sub-scanning direction.
By implementing the correction process when the power is turned on, when environment such as temperature changes, or when printing is performed more than a predetermined number of copies, control is provided so that each amount of the color shifts is always in a predetermined range or less. Among the amounts of the color shifts, the registration shifts in the main and the sub-scanning directions can be corrected by controlling a timing of laser-beam writing in a photosensitive element, and the main-scanning magnification shift can be electrically corrected by controlling pixel clocks.
As for skew of the laser beam used for scan and exposure, there are two methods: a method of mechanically correcting the skew and a method of correcting an output image through an image process. In the method of mechanically correcting the skew, a control mechanism that displaces a mirror inside a laser-beam writing unit is provided to implement the correction. However, an actuator such as a mirror displacement motor is required to automatically implement the correction, which causes an increase in cost and prevents reduction of the size of the laser-beam writing unit.
Meanwhile, in the method of correcting the output image through the image process, part of an image is stored in a line memory and the image is read from the line memory while switching a read position from one to another so as to shift the output image in an opposite direction to the skew and output the image, so that the skew between the colors is corrected. This case has an advantage of achieving the correction with comparatively low cost as compared with the mechanical correction because the line memory is simply added to an image processing unit according to a correction range.
Japanese Patent No. 3715349 discloses an image recording device that reduces the skew through the image process. The image recording device can reduce stripe-like noises by detecting whether a target pixel is at a shift position and by providing density correction to the target pixel when a change point of an adjacent pixel in the main-scanning direction and a pixel pattern around the target pixel match a predetermined pattern at the shift position. More specifically, the density correction corresponds to a set of the pixel pattern and the change point of the adjacent pixel in the main-scanning direction matching the predetermined pattern.
However, in the technology described in Japanese Patent No. 3715349, a fixed density correction value is always only added or subtracted to or from the target pixel or the pixel in which noise (density shift) occurs. In other words, the density correction is not always provided to the pixel so as to cancel out density variation. Therefore, the stripe-like noises cannot sometimes be appropriately corrected.